Presents your  
JAVA E-NEWSLETTER for October 23, 2003 
 
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SEALING YOUR JARS CAN SOLVE SOME OF YOUR CLASSPATH PROBLEMS   
  
Many Java-induced headaches are caused by the classpath. One of the 
problems with the classpath is that multiple versions of the same class or 
classes can be in the classpath at the same time. The virtual machine will 
load the first one it finds. There are times, such as when your 
application is running in a container, when this situation cannot be avoided. 
Sealed jars offer a partial remedy for this problem.   
  
When you use sealed jars in your application, the class loader will 
ensure that all classes in the same package are loaded from the same jar
file. For example, say that class com.acme.A is loaded from the sealed file 
acme.jar. Later, the class loader is told to load the class com.acme.B. 
Since A and B are in the same package, the class loader attempts to load
class B from acme.jar. If class B cannot be found in acme.jar but can be
found elsewhere, then a SecurityException is thrown.   
  
Sealing a jar is as simple as providing an entry in the jar's manifest 
file. To create a sealed jar that contained all of the classes in the com 
directory and its subdirectories package, here's how the command would look:   
  
jar cmf mymanifest.txt myjar.jar com   
  
where mymanifest.txt contains:   
  
Manifest-Version: 1.0   
Sealed: true   
  
The whole jar is now considered sealed by the class loaders that use it.

  
Sealing your jars won't solve all of your classpath problems, but it can
be helpful. This is also a good habit to develop, so start making sealed
jars part of your deployment system. There are more options for sealing;
for instance, you can seal some packages and not others. You can find 
out more here.   
http://java.sun.com/j2se/1.4.2/docs/guide/extensions/spec.html   
  
David Petersheim is a Senior Java Developer with Genscape, Inc. He 
designs and develops server-side applications to acquire and process
real-time energy data.  
 
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